One well known way to increase the performance of hard disk drives is to increase the areal data storage density of the magnetic hard disk. This can be accomplished by reducing the written data track width, such that more tracks per inch can be written on the disk. To read data from a disk with a reduced track width, it is also necessary to develop sufficiently narrow read head components, such that unwanted magnetic field interference from adjacent data tracks is substantially eliminated.
The ability to develop and deliver sub 130 nm trackwidth read sensors critically depends upon the ability to ion mill the sensor and to reliably lift-off the deposited stabilization (hard magnet) and lead materials. As shown in FIG. 1, the ion mill used in manufacturing for trackwidth or stripe height definition of a sensor 100 uses a single mill step at high incidence angle (0 to 15 degrees from normal incidence, i.e., perpendicular to the plane of the surface being milled). State of the art manufacturing processes require a photoresist bilayer 102, which defines the milling. The bottom layer 104 of the photoresist bilayer 102 is undercut to a finer dimension than the trackwidth W. This undercut prevents material from redepositing on top of the track during the ion mill step and is critical for the lift off process. The high incidence milling usually creates sharp mill profiles and if a bilayer photoresist is used, redeposited metal from milling accumulates in the undercut 106 and is removed during lift-off. No redeposited material remains at the junction edge.
However, as the sensor trackwidths have decreased with product and technology roadmaps, the bilayer resist process has reached its limits. It cannot be performed reliably for trackwidths below 130 nm, mainly because the width of the photoresist underlayer becomes too small to support the overlayer. One solution is to use a resist structure without undercut. Such structure can be a single layer resist or a multilayer provided that no undercut is formed. In the case of a multilayer, the resist image can be patterned by lithography or dry etching techniques such as ion mill, reactive ion etching (RIE), etc. Using a single layer photoresist is advantageous because only one photoresist image must be controlled and reliable lift-off can still be performed using chemical-mechanical polishing (CMP) based lift-off. Particularly, by using a single layer photoresist, no undercut is formed, resulting in a stable photoresist structure. Further, the photoresist and much of the redeposited material coupled to it can be easily removed.
While using a single layer photoresist (without undercut) milled at high incidence, the milling profile is sharper but redeposited metal from the mill accumulates on the resist side walls and also on the side walls of the structure being milled, such as a sensor. Elemental analysis of the junction side has shown that sensor material and alumina (from gap) are redeposited. This unwanted material creates a large physical separation between the read sensor and its stabilization (HB) layer, resulting in increased sensor resistance and poor magnetic stability.
Referring to FIG. 2, in a typical process, after milling to form the sensor 200, a thick seed layer (e.g., Cr or other suitable material) 202 is formed, and then a HB layer 204 is formed over the seed layer. The single layer photoresist 206 can form the mask for deposition of the seed and HB layers. What is important for magnetic properties of a sensor is the distance between the free layer and the HB layer. Ideally, the HB layer 204 and free layer of the sensor 200 are aligned, but if the separation between the HB and free layers is too large, the HB layer 204 will not form a magnetic junction with the free layer. Milled material 208 redeposited on the sides of the sensor increases the separation between the free layer and the HB layer 204. The increased separation translates into free layer instability. Note also that the thicker the sensor 200 is going to be, the thicker the residual material 208 that forms, compounding the problem. Thus, it would be desirable to remove a majority of the redeposited material from the sensor sidewalls inherently formed in a single layer photoresist milling process.
In addition, when liftoff is performed, residual material (fencing) 208 remains on the wafer surface. FIG. 3 illustrates the structure of FIG. 2 after liftoff of the photoresist 206. Because the component will typically be covered with another material, the fencing can create shorts. For example, if a magnetic sensor is being formed, a dielectric gap layer 302 is placed on top of the wafer, and a shield layer 304 (e.g., a metal such as NiFe, etc.) is added above the gap layer 302. A typical gap layer 302 is only about 150 Å thick, and will be very thin or nonexistent along the peaks of the fencing. The result is that current passes from the sensor 200 via the fencing through the thin gap layer 302 to the shield layer 304, causing a short. Thus, it would be desirable to reduce fencing formed during a single layer photoresist milling process.
Another problem is that fencing causes a flaring of the gap. The gap is the distance between the shield layers. As known to those skilled in the art, the perfect gap Gi is of a predetermined thickness. The actual magnetic track width of the sensor height is much larger than the physical track width because the fencing adds to the gap (now Gi, GA, GB), making it uneven (see FIG. 3). The result is that the sensor has a much different magnetic proportion than the design ideals, leading to inferior performance. Thus, it would be desirable to reduce flaring formed during a milling process using a resist structure without undercut.